This invention relates generally to a technique for optimizing stabilizer positioning in a bottomhole assembly or BHA. More particularly, this invention relates to the optimization of stabilizer positioning in a BHA so as to eliminate the effects of borehole inclination.
It is well known that numerous factors determine the behavior of a BHA. Some of the more important of these factors include:
1. BHA design, i.e., stabilizer location and size as well as collar size PA1 2. Borehole inclination and curvature PA1 3. Borehole size PA1 4. Weight on Bit (WOB) PA1 5. Rotations per minute (RPM) PA1 6. Bit and stabilizer side cutting PA1 7. Formation dip and anisotropy PA1 1. Dynamical effects of rotary speed (Item 5) PA1 2. Changes in hole size due to bit and stabilizer side cutting, hole washout and mechanical erosion (Item 6) PA1 3. Changes in hole inclination because of formation dip and anisotropy (Item 7)
Typically, a majority of rotary bottomhole assemblies for directional control are designed through practical experience and trial and error. This approach can produce satisfactory results when a great deal of local experience can be drawn on. However, in drilling new areas, the use of trial and error can prove costly because of the increased number of trips and correction runs. Mathematical models are known for predicting the directional inclination tendencies of rotary assemblies which are helpful in limiting the uncertainty associated with the traditional BHA design techniques. Most of these mathematical models are two dimensional and static. Examples of these mathematical models are described in Walker, B. H. and Frieman, M. B.: "Three-Dimensional Force and Deflection Analysis of a Variable Cross Section Drillstring", J. Pressure Vessel Tech. (May 1977) 367-73; Murphy, C. E. and Cheatham J. B. Jr.,: "Hole Deviation and Drill String Behavior", SPEJ (March 1966) 44-49; Trans., AIME, 237; Callas, N. P. and Callas, R. L.: "Stabilizer Placement," Oil and Gas J. (Nov. 24, 1980) 142-52; (Dec. 1, 1980) 140-55; (Dec. 29, 1980) 186-90; Jogi, P. N., Burgess, T. M., and Bowling J. P.: "Predicting the Build/Drop Tendency of Rotary Drilling Assemblies," SPEDE (June 1988).
While the above described mathematical models primarily take into account Items 1-4 as discussed above, discrepancies do occur due to:
If dynamical effects due to RPM are ignored, then for a given BHA design, a given weight-on-bit and a given mud weight, the BHA response is only dependent on hole inclination, formation dip and changes in borehole size. Therefore, if the hole angle dependency is eliminated from the above list, any discrepancy in the results will then be a function of only the hole size changes (Item 6) and dip angle changes (Item 7). Since as mentioned, the majority of rotary bottomhole assemblies are designed using practical experience and trial and error, and since response of all assemblies is dependent on borehole inclination, assemblies must be changed more often. This problem would be alleviated if a method could be found for designing a BHA which eliminates the effect of borehole inclination.